Format-Specific Patterns

Format-Specific Patterns

Practical dasel v3 patterns for each supported data format, plus conversion caveats.

JSON Patterns

Nested Object Queries

# Deep path access
cat package.json | dasel -i json 'dependencies.react'

# Array of objects -- get all names
cat data.json | dasel -i json 'users.map(name)'

# Filter and extract
cat data.json | dasel -i json 'users.filter(role == "admin").map(email)'

Array Manipulation

# Get array length
cat data.json | dasel -i json 'len(items)'

# Slice first 3
cat data.json | dasel -i json 'items[0:2]'

# Sum numeric field
cat data.json | dasel -i json 'orders.map(total).sum($this)'

Modify JSON

# Set nested value
cat config.json | dasel -i json --root 'settings.theme = "dark"' > config_new.json

# Add to array
cat data.json | dasel -i json --root 'tags = [tags..., "new-tag"]'

# Remove key by reconstruction
echo '{"keep": 1, "drop": 2, "also_keep": 3}' \
  | dasel -i json --root '{ keep, also_keep }'

Recursive Search in JSON

# Find all objects with "id" field anywhere in tree
cat complex.json | dasel -i json 'search(has("id"))'

# Find all string values
cat data.json | dasel -i json 'search(typeOf($this) == "string")'

YAML Patterns

Basic Queries

# Read nested key
cat config.yaml | dasel -i yaml 'database.host'

# Read list item
cat config.yaml | dasel -i yaml 'servers[0].name'

# Get all server names
cat config.yaml | dasel -i yaml 'servers.map(name)'

Modify YAML

# Update a value
cat config.yaml | dasel -i yaml --root 'database.port = 5433' > config_new.yaml

# YAML preserves structure when using same input/output format
cat config.yaml | dasel -i yaml -o yaml --root 'app.debug = false'

Multi-Document YAML

dasel processes the first document in a multi-document YAML stream by default. To work with specific documents, split them first or pipe individual documents:

# Process first document
cat multi.yaml | dasel -i yaml 'metadata.name'

YAML Anchors and Aliases

dasel resolves YAML anchors/aliases before query execution. Queries operate on the resolved values, not the anchor references:

# Given YAML with anchors:
# defaults: &defaults
#   timeout: 30
# production:
#   <<: *defaults
#   timeout: 60

cat config.yaml | dasel -i yaml 'production.timeout'
# Output: 60 (resolved value, not anchor)

TOML Patterns

Table Access

# Read from a table
cat config.toml | dasel -i toml 'database.server'

# Read from nested table
cat config.toml | dasel -i toml 'servers.production.host'

Arrays of Tables

# TOML [[products]] becomes an array
cat config.toml | dasel -i toml 'products[0].name'

# Filter array of tables
cat config.toml | dasel -i toml 'products.filter(price > 10)'

# Get all product names
cat config.toml | dasel -i toml 'products.map(name)'

Modify TOML

# Set value in TOML table
cat config.toml | dasel -i toml --root 'database.port = 5432' > config_new.toml

# TOML type preservation
cat config.toml | dasel -i toml --root 'settings.max_connections = 100'

Inline Tables

# Access inline table values
# Given: point = { x = 1, y = 2 }
dasel -f data.toml 'point.x'

XML Patterns

XML has two parsing modes controlled by --read-flag xml-mode=structured:

• Friendly mode (default): Attributes stored with - prefix (e.g., -id), text content as #text. Source: parsing/xml/reader.go:98-165
• Structured mode: Explicit keys: name, attrs, content, children. Source: parsing/xml/reader.go:58-96

XML Security Limits

Source: parsing/xml/reader.go:15-21

• Maximum input size: 10 MB (maxXMLSize = 10_000_000)
• Maximum bytes per comment: 10 KB (maxCommentLength = 10_000)
• Maximum comments per document: 1,000 (maxTotalComments = 1_000)

Element Access

# Read element text
echo '<root><name>Alice</name></root>' | dasel -i xml 'root.name'

# Nested elements
cat data.xml | dasel -i xml 'catalog.book[0].title'

# All book titles
cat data.xml | dasel -i xml 'catalog.book.map(title)'

Attributes (Friendly Mode)

XML attributes are accessed via a special - prefix convention in friendly mode:

# Access attribute
# Given: <item id="123">content</item>
echo '<root><item id="123">content</item></root>' \
  | dasel -i xml 'root.item.-id'

# Filter by attribute
cat data.xml | dasel -i xml 'items.item.filter(-type == "active")'

Text Nodes (Friendly Mode)

# Element text content accessed directly (simple elements)
echo '<root><message>Hello World</message></root>' \
  | dasel -i xml 'root.message'
# Output: Hello World

# Mixed content: text accessible via #text key
# Given: <item id="1">content</item>
echo '<root><item id="1">content</item></root>' \
  | dasel -i xml 'root.item.#text'

Structured Mode

# Access with structured mode
echo '<root><item id="1">content</item></root>' \
  | dasel -i xml --read-flag xml-mode=structured 'root.children[0].attrs.id'

# Structured mode keys: name, attrs, content, children
echo '<root><item id="1">content</item></root>' \
  | dasel -i xml --read-flag xml-mode=structured 'root.children[0].content'

Modify XML

# Set element value
cat data.xml | dasel -i xml --root 'config.timeout = 30' > data_new.xml

CSV Patterns

Column Access

CSV data is treated as an array of arrays (or array of objects if headers present):

# Access by row and column index
echo 'name,age
Alice,30
Bob,25' | dasel -i csv '[0].name'
# Output: Alice

# Get all values from a column
echo 'name,age
Alice,30
Bob,25' | dasel -i csv 'map(name)'
# Output: ["Alice", "Bob"]

Row Filtering

# Filter rows by column value
echo 'name,age,active
Alice,30,true
Bob,25,false
Carol,35,true' | dasel -i csv 'filter(active == "true")'

# Note: CSV values are strings; compare as strings
echo 'name,score
Alice,90
Bob,75' | dasel -i csv 'filter(toInt(score) > 80)'

CSV to JSON

# Convert CSV to JSON array of objects
cat data.csv | dasel -i csv -o json

HCL Patterns

HCL (HashiCorp Configuration Language) support was added in v3.0.0.

Block Access

# Read resource block
# Given HCL:
# resource "aws_instance" "web" {
#   ami = "ami-12345"
#   instance_type = "t2.micro"
# }
cat main.tf | dasel -i hcl 'resource.aws_instance.web.ami'

Labels

HCL blocks with labels are accessed as nested maps:

# Access by block type and label
cat main.tf | dasel -i hcl 'variable.region.default'

# List all resource types
cat main.tf | dasel -i hcl 'keys(resource)'

Nested Blocks

# Access nested block
cat main.tf | dasel -i hcl 'resource.aws_security_group.web.ingress[0].from_port'

INI Patterns

INI support was added in v3.0.0.

Section and Key Access

# Read key from section
# Given INI:
# [database]
# host = localhost
# port = 5432
cat config.ini | dasel -i ini 'database.host'
# Output: localhost

cat config.ini | dasel -i ini 'database.port'
# Output: 5432

Modify INI

# Update a value
cat config.ini | dasel -i ini --root 'database.port = 3306' > config_new.ini

Global Keys

Keys outside any section are accessed at the top level:

# Given INI:
# version = 2
# [server]
# host = localhost
cat config.ini | dasel -i ini 'version'

Format Conversion Matrix

Conversion Syntax

# Generic pattern
cat input_file | dasel -i <input_format> -o <output_format>

# JSON to YAML
cat data.json | dasel -i json -o yaml

# YAML to TOML
cat config.yaml | dasel -i yaml -o toml

# TOML to JSON (compact)
cat config.toml | dasel -i toml -o json --compact

# CSV to JSON
cat data.csv | dasel -i csv -o json

Conversion Caveats

XML conversions:

• XML attributes (-attr) may not round-trip cleanly to JSON/YAML/TOML
• XML text content mixed with child elements requires careful handling
• XML namespaces are not preserved in conversion

CSV limitations (source: parsing/csv/writer.go:16, parsing/csv/csv.go:24):

• Root value must be a slice/array for CSV writing: "csv writer expects root output to be a slice/array"
• CSV to structured formats works only for flat tabular data
• Nested structures in JSON/YAML/TOML cannot represent well as CSV
• All CSV values are strings; numeric/boolean types must be cast after conversion
• Custom delimiter via --rw-flag csv-delimiter=; (source: parsing/csv/csv.go:24)

TOML limitations (source: parsing/toml/toml_writer.go:28-29):

• TOML does not support null values; null values from JSON/YAML are dropped or converted to empty strings
• TOML requires all keys to be strings
• TOML datetime types may not convert cleanly to other formats
• Does not preserve formatting metadata (multiline strings, etc.) on round-trip

INI limitations (source: parsing/ini/ini_writer.go:26-28, 73-74, 88-89):

• Root must be a map: "ini can only represent map values"
• Cannot represent slice/array values directly: "ini writer cannot represent slice values directly"
• Sections can only represent map values
• All values are stored as strings

HCL limitations (source: parsing/hcl/writer.go:49-50):

• Body must be a map: "hcl body is expected to be a map"
• HCL and INI conversion to other formats may lose format-specific semantics (HCL block labels, INI section structure)
• Round-trip fidelity is best when staying within the same format

YAML notes (source: parsing/yaml/yaml_reader.go:24-54):

• Multi-document YAML is supported: single document returns the value directly; multiple documents return a branched slice
• Empty YAML returns null

Extended Format Options

Format-specific behavior can be customized via --read-flag, --write-flag, and --rw-flag. Each accepts name=value pairs.

Source: internal/cli/read_write_flag.go, internal/cli/query.go:7-9

# CSV with semicolon delimiter
echo 'a;b;c\n1;2;3' | dasel -i csv --rw-flag csv-delimiter=';' 'map(a)'

# XML structured mode
cat data.xml | dasel -i xml --read-flag xml-mode=structured 'root.children[0].name'

# HCL array block format
cat main.tf | dasel -i hcl --read-flag hcl-block-format=array 'resource'

Sources

• dasel README: https://raw.githubusercontent.com/TomWright/dasel/master/README.md (fetched 2026-02-19)
• dasel docs: https://daseldocs.tomwright.me (fetched 2026-02-19)
• Arrays/slices: https://daseldocs.tomwright.me/syntax/arrays-slices.md (fetched 2026-02-19)
• Objects/maps: https://daseldocs.tomwright.me/syntax/objects-maps.md (fetched 2026-02-19)
• CHANGELOG (format support): https://raw.githubusercontent.com/TomWright/dasel/master/CHANGELOG.md (fetched 2026-02-19)
• Source code analysis: parsing/xml/reader.go, parsing/csv/writer.go, parsing/csv/csv.go, parsing/ini/ini_writer.go, parsing/hcl/writer.go, parsing/toml/toml_writer.go, parsing/yaml/yaml_reader.go, internal/cli/read_write_flag.go (analyzed 2026-02-19)